Posted By:
Kevin CummingsI think several of the posts have pretty much answered the question. It's really just tradition, habit and evolution, isn't it?
In the early days of baseball, the "captain" was quite often a playing member of the team who was responsible for the on field decisions. Most teams may have had someone serving as "manager," but their job was to take care of administrative things like travel arrangements and salaries.
After the turn of the century, those guys who had formerly been top players and perhaps captains of their teams, but were on the down side of their playing careers began to be sought after to simply direct the players on the field. Having just finished reading Ed Delahanty and the Emerald Age of Baseball this was one of the key points of emphasis. While Connie Mack was a clear exception to wearing the uniform to perform this function, people like John McGraw and Hughie Jennings were not.
So the 19th century "captain" evolved into the 20th century "manager" and just continued to wear his 19th century work clothes to do his job, no?